This invention relates to fixtures employed in testing features on a first surface of a device under test, which has an opposite surface; and more particularly to means for isolating the opposite surface of a semiconductor device under test from fluids retained in contact with the first surface.
In the past, measurement of features of Devices Under Test (DUT)s by Scanning Acoustic Microscopy (SAM) and C-mode Scanning Acoustic Microscopy (C-SAM) has been performed by immersion of a workpiece in an amplification medium comprising an acoustic transmission fluid, e.g. alcohol, oil, or water, to transmit the sound waves necessary for the measurement. Copending, commonly assigned U.S. patent application Ser. No. 11/495,243 of Lu entitled “Imaging Thin Film Structures by Scanning Acoustic Microscopy” describes C-SAM testing apparatus used for testing devices with one side exposed to an acoustic transmission fluid, e.g. water, and the other side isolated from the transmission fluid in a space filled with air. In the past, testing with C-SAM apparatus has involved complete immersion of the workpiece in an amplification medium (alcohol, oil, water) to transmit the sound waves necessary for the measurement.
Whereas an acoustic transmission fluid is preferred as a transmitter of acoustic waves, there is a problem that the acoustic transmission fluid should not be in contact with the front surface of a DUT. The solution described by the Lu application is to immerse the back surface of the DUT and to isolate the features on the front surface of the DUT by employing an impervious barrier structure. The impervious barrier structure retains the acoustic transmission fluid in contact with the back surface of the DUT. The barrier excludes the acoustic transmission fluid from the front surface of the DUT. That is to say that the front surface of the DUT is located in a vacuum or in a gas and is isolated from the acoustic transmission fluid, either by providing a sealed chamber protecting the front surface, or by providing a dam, while the acoustic transmission fluid is retained in contact with the back surface of the DUT.
U.S. Pat. No. 4,932,358 of Studley et al entitled “Perimeter Wafer Seal” presses a seal ring (preferably composed of monel metal) against a wafer on a CVD chuck around the outer periphery of the wafer with sufficient force to hold the backside of the wafer against the chuck. Thus no CVD material may deposit on the backside of the wafer. The seal ring has one surface for contacting the front side of the wafer and a second surface that extends close to the CVD chuck, so the edge of the wafer is protected from CVD coating. Thus CVD coating is confined to the front side of a wafer. In a preferred embodiment, an apparatus with a slide operated by a cam lever and a tension spring moves the seal ring and presses it against a wafer with multiple chucks attached to a rotatable turret within a CVD chamber.
U.S. Pat. No. 5,421,401 of Sherstinsky et al. entitled “Compound Clamp Ring for Semiconductor Wafers” uses a compound clamp ring to secure a semiconductor wafer to a wafer pedestal during wafer processing to prevent leakage of coolant gases circulated at the backside of the wafer into the process environment. The clamp ring seals against application of back side gas pressure against wafers to ensure thermal transfer during plasma processing, but it does not provide a wet fluidic seal, nor does it offset Z-forces against a wafer at points of wafer seal contact, and it expressly avoids the use of elastomers because of possible outgassing in a vacuum chamber at high temperatures.
U.S. Pat. No. 6,244,936 of Kao et al entitled “Method and device for Reducing Semiconductor Defects Caused by Wafer Clamping” describes reducing defects in semiconductor wafers caused by a wafer clamp ring by polishing the surfaces of the clamp ring that engage and apply clamping force to the wafer. A polishing tool includes a circular plate supported on the stationary base. A layer or pad of polishing material, such as silicon carbide diamond, is deposited over the plate. The clamp ring is placed on the plate such that clamping surfaces of the ring engage the polishing material on the plate, and the ring is rotated to effect polishing of the clamping surfaces.
U.S. Pat. No. 6,523,426 of Vincent et al. entitled “Water Quality Measuring Apparatus with a Sensing Wafer Clamped Between Two O-Rings” describes a square wafer which measures the quality of water by detecting electroactive elements or compounds present in a solution, e.g. dissolved oxygen, pH, temperature, and chlorine and ammonia levels. The wafer is located between O-Rings on the top and bottom sides thereof so that the wafer is exposed to substantially equal pressure on both sides. Central portions of the wafer are exposed to equal amounts of atmospheric pressure and remaining peripheral portions of the wafer are exposed to equal amounts of water pressure.